selftests: netfilter: nft_fib.sh: fix spurious test failures

selftests: netfilter: nft_fib.sh: fix spurious test failures

Jakub reports spurious failure of nft_fib.sh test.
This is caused by a subtle bug inherited when i moved faulty ping
from one test case to another.

nft_fib.sh not only checks that the fib expression matched, it also
records the number of matches and then validates we have the expected
count.  When I did this it was under the assumption that we would
have 0 to n matching packets.  In case of the failure, the entry has
n+1 matching packets.

This happens because ping_unreachable helper uses "ping -c 1 -w 1",
instead of the intended "-W".  -w alters the meaning of -c (count),
namely, its then treated as number of wanted *replies* instead of
"number of packets to send".

So, in some cases, ping -c 1 -w 1 ends up sending two packets which then
makes the test fail due to the higher-than-expected packet count.

Fix the actual bug (s/-w/-W) and also change the error handling:
1. Show the number of expected packets in the error message
2. Always try to delete the key from the set.
   Else, later test that makes sure we don't have unexpected keys
   in there will always fail as well.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netfilter-devel/20250927090709.0b3cd783@kernel.org/
Fixes: 98287045c979 ("selftests: netfilter: move fib vrf test to nft_fib.sh")
Signed-off-by: Florian Westphal <fw@strlen.de>